Aquinas wrote in the Summa Theologica: "We use the "is" verb to signify both the act of existing and the mental uniting of a predicate to subject which constitutes a proposition. " Here the Angelic Doctor makes the classical mistake of investing "is" with a copuletic subroutine in addition to endowing the word with a misconceived function denoting existence. Thomas Aquinas talks at great length about a view of the verb which he says the ancient grammarians did not mention: the view that every verb is capable of being reduced, in terms of logic, to a form containing the verb 'be' so that: "A dog runs" can be expressed as "a dog is running. " This process of logical translation, called 'compositio,' was thought to display the real structure of the verb and became a commonplace of scholastic logic. It is the re-introduction of the copula, masquerading rather half-heartedly as a quasi-grammatical category. The frequent tag, smoothing over the difference between the logical and the grammatical approaches, was to call the verb 'be' (the radix omnium verborum, ' [the root of all words] an expression which appears again in Sanctius. The renaissance grammarians, like their classical predecessors followed the example of Aquinas unquestioningly, neglecting to see that the 'is' processes the 'substantiality, ' which is wholly established by the subject of the sentence, into what is predicated of it. But the notion of the "verbum substantivum" is obviously heavily tied in with the idea of 'substance' separable from 'matter, ' a separation that is made impossible when 'substance' is always the specific substance of what is introduced in the subject.

When we arrive at the sixteenth and seventeenth centuries it is difficult not to feel smothered by the fog of picayune minutia which is stirred up when a category such as the verb is studied intimately. But a detailed scrutiny of the various grammar books written at that time does bring out very distinctly the fact that the classification of 'be' as a verb was radically unstable.
Under the suffocating influence of the Latinate tradition most of the contemporary grammarians thought that the copula was firmly established as a verb; that they nonetheless treated it in a great variety of ways (far greater than they themselves realised.)

William Ward.

William Ward writing in his Grammar of 1765 while admitting that 'to be' was a neuter verb would not grant that it was the type and pattern of all neuter verbs.
"Verbs intransitive or neuter as they are sometimes called he defined characteristically as:
"Verbs in which it is not easy to distinguish the state as it is in exertion from the state as it is in the reception of the effect produced by such exertion… verbs which denote neither action nor suffering or states which are intermediate between both [e. g.] to grow, to wither, to rest, to exist. He then explains the peculiar status of be:

"The verb 'to be' is manifestly a verb neuter; but it differs from all other verbs in this very considerable circumstance, viz. that it admits of every noun substantive to depend upon it in immediate apposition: whilst by such apposition a connexion is denoted between the state signified by it, and the object signified by the substance on which it depends; yet no new state as in the substantive."
(W. Ward, 1765, pp. 176-7.)

By this perceptive observation Ward presumably means (the illustration is not his) that in. "John walks" the verb walks adds something to the idea conveyed by John; John is in the 'new state' of walking. But in "John is" the verb adds nothing, because John must already 'be' in a state of existence; the verb 'to be' can add no' new state' to an already existing subject. Ward complicates his explanation of a simple point by speaking at one moment as if John ' depended on' walked, and at the next as if walked depended on John, and by referring simultaneously to the two words as being in apposition, which should mean that neither the other. It is difficult not to feel suffocated by the cloud of trivial detail that is stirred up when a category such as the neuter verb is examined at all closely. But a detailed examination does bring out very clearly certain facts that most of the early grammarians thought they were a firmly established category; that they nevertheless treated it in a great variety of ways (far greater than they themselves realised) that the category was radically unstable.